The value of the contact resistance between the metallic current lead stripe and the composite coating is among the key parameters of chemical power sources including batteries, supercapacitors, photovoltaic modules, and fuels cells.
This coating is formed by using powdered materials that have ionic conductivity, and additives of carbon, as well as soot and other materials in different combinations. The selected combinations of materials are subsequently fixed with a binder. For example, aluminum, copper, nickel, and titanium are usually used as the metal for the current leads. The specific electrical conductance of the coating is lower than that of the metal current lead by several orders of magnitude.
The value of the contact resistance between the current lead and the coating is a function of the ohmic contact area. The latter is determined by the micro-relief of the coating surface, as well as by the presence and the total area of an insulating film of aluminum oxide on the surface of the current lead. When the coating is being applied to the current lead stripe, the oxide film is substantially destroyed, while for some coatings (thin coatings of high porosity) it remains possible to partially restore this film due to the oxygen diffusion from the air through the coating layer, as well as due to the oxygen diffusion from the interior pores of the coating.
In order to determine the resistance between the current lead stripe and the coating on the fabrication line while the stripe is in motion it is necessary to use non-contact measurement methods. Any mechanical contact with the surface of the coating while the stripe is in motion would lead to the destruction of the highly porous surface of the coating at the point of contact. This effect would be exacerbated by the small thickness of the coating (10-150 μm) and the relatively low density of the coating material (1.5-3.0 g/cm3).
A destructive testing method that includes cutting of test samples from finished stripe with a coating, and subsequent determination of the transient resistance between the foil and the coating by a contact method is not practical due to the following.
First, to meet the requirement of making the sampling representative, the number of test samples has to be substantial, and the samples have to be cut from different regions of the stripe. To make the test samples representative they must be removed from areas that are uniformly distributed over the entire stripe surface. The cutting out of the samples will detrimentally affect the integrity of the stripe and substantially complicate the subsequent process of fabricating the final products such as chemical power sources and supercapacitors.
Second, the contact method of measuring the specific resistance of the coating material and of the transient resistance between the foil of the current lead foil and the coating has inherent problems. For example, the coating is highly porous. Its porosity usually equals more than 20 percent, and in some cases it exceeds 50%.
Due to the presence of substantial open porosity, the surface of the coating layer features a rather complicated stochastic micro-relief. By pressing the contact area of the measuring electrode to this surface the micro-relief of the surface will be disturbed. In addition, the area of the actual ohmic contact of the coating and of the contact region will be different at each measurement thus introducing substantial errors into the measurement process.
Among the non-contact testing methods for determining the resistance between the current lead stripe and the coating, the methods of eddy currents and electrical capacitance appear, in principle, to be the best suited.
Let us review the possibility of using the non-contact electrical capacitance method. The non-contact electrical capacitance method involves using two or more coplanar wafers placed above the coating with a certain air gap, as shown in FIG. 1. The interaction between the potential electrical field of such a strap capacitor and the object being tested is well reflected by an equivalent circuit shown in FIG. 2 for a capacitor comprised of two wafers.
Since the dimensions of the wafers in the strap type capacitor substantially exceed the distance between the surfaces of wafers 101 (FIG. 1) and the surface of the aluminum foil 104, a circuit of two plane-parallel series capacitors, is actually formed.
Each of these capacitors is formed by the corresponding plate of the strap capacitor 101 and by the field-screening surface of the aluminum foil. The connection of the wafers of these formed capacitors are effected in the foil, and the connecting point of these capacitors is schematically shown at an equivalent circuit (FIG. 2), and designated as “foil”.
Each of these plane-parallel capacitors contains three layers arranged in series in the field between its wafers. The capacitance of the first layer, formed by the air gap between the capacitor wafer and the coating surface, is designated on the equivalent scheme (FIG. 2) as Cg (201). The second layer comprising a low-conductance coating is shown on the equivalent circuit as a chain of parallel-connected resistors Rm (203) and Cm (204). The third layer is comprised of the Al2O3 dielectric film formed on the surface of the aluminum foil. In the equivalent circuit scheme it is represented by the capacitor Cd (202).
The thickness of the Al2O3 film is rather small, usually not exceeding ten nanometers. Therefore the value of the capacitor Cd is higher by 3-4 orders than that of the capacitors Cm and Cd. Correspondingly, the capacitive reactance of the oxide film is close to zero. Therefore it is practically impossible to determine the presence or, absence of an oxide film from the change of the total capacitance value of the strap capacitor.
In certain cases a strap capacitor with coplanar wafers can be used for determining the distance from its electrodes to the conducting surface of a sample. So, for example, according to the U.S. Pat. No. 6,593,738, for measuring the thickness of thin conducting coatings on various structures, in particular of metallic films on dielectric or semiconductor disks, the eddy current method is used. However, the eddy current probe in this case is mechanically connected to an electrical capacitance probe, which is used for determining the distance to the surface of the metallic film.
The non-contact method of eddy currents is used for measuring electromagnetic characteristics of the conducting layers and of the coatings, including thickness, and for detecting faults in the layers.
In the literature some authors proposed the use of a multiple frequency method of eddy currents for detecting corrosion in two-layer structures. They note that the eddy current method has been widely used for detecting subsurface discontinuities in the structures of aircraft. In the work described, the coil is excited by a sinusoidal alternating current (the typical frequency range is from 50 Hz to 5 MHz), which induces eddy currents in the electrically conductive material. The coil impedance is then measured.
Discontinuities, such as fissures, corrosion and surface characteristics lead to changes in the amplitude and phase of the eddy current. At the same time the eddy current signal can be distorted by a number of interfering factors that drastically complicate interpretation of the signal. With regard to multiple layer structures, such as aircraft splice joints such factors as probe deflection and skewness due to the surface deformation, change of paint thickness, simultaneously occurring discontinuities, and changes in the gap between the layers can all affect the measurements and complicate signal interpretation.
Single-frequency testing provides little information for reducing the influence of these factors. At the present time, multiple frequency eddy current testing has been successfully used for detecting hidden corrosion and subsurface cracking in aircraft lap splices. In the literature the use of the eddy current method for determining the material loss as a result of corrosion in a two-layer structure while using four operating frequencies, namely; 30 kHz, 17 kHz, 8 kHz, and 5.5 kHz is described.
The eddy current probe that rotates in a hole for fixing a multiple layer structure is described in literature. The probe output signal subject to filtration is used for detecting the presence of defects, while the probe signal not subject to filtration is used for determining the boundaries between the layers in a multiple layer structure.
The literature also describes eddy current inspection systems for nondestructive detection of faults in the region of a multiple layer conducting structure in the place of the junction of the structure. In the first of them the eddy currents are induced in the structure by an aperiodic excitation current in the induction coil. Excitation and receiving coils are used, while a change in the height of the altitude connection and of the gaps between the intermediate conducting layers in the multiple layer structure is being compensated. In this case the eddy current transducer is excited by a pulsed current while the received signals are being filtered. The presence of the air gap between the layers of the structure is indicated. As a result of filtration the parameters of the intermediate layer, the thickness of the structure being inspected are determined. Quantitative measurements of the defect parameters are obtained by comparing the signal value with the calibration curve. The Fourier transformation of the signals is used for obtaining the amplitude-frequency characteristics. A frequency filtration is used.
A device and a process for nondestructive determination of changes of the ohmic resistance of a thin layer by using eddy currents have also been described. The induction coil is excited by a high frequency current. Its magnetic field induces eddy currents in the layer thus weakening the magnetic field of the coil. The coil forms a part of the generating circuit that is constantly maintained in the resonance by using phase correcting circuits. Under such resonant conditions, the reactive parts of the generating circuit may be neglected, and the current flowing in the generating circuit depends exclusively on the ohmic resistance of the induction coil, the changes in which determined by the ohmic resistance of the layer being tested.
The use of a parallel resonant circuit connected to the harmonic oscillations generator for measuring the electrical conductance of the media is also described in patent FR2782802A1.
Other researchers have described a method for eddy current testing of a minimum of one layer placed on a substrate. A minimum of one layer or the substrate conducts electric current. An inductance coil is used as a primary field source or to measure the secondary field parameters arising from the eddy currents induced in the conducting layer or in the substrate. The primary magnetic field is generated at a minimum of two frequencies. The measured values of the added impedance are used for determining the electromagnetic properties of the substrate and the layer, as well as the layer thickness.
An eddy current method for testing multiple layer coarse grain weld seams for the absence of defects has also been described in literature. It is noted that the detection of cracks in a weld seam is complicated due to the coarse grain material in the seam. U.S. Pat. No. 6,524,460 describes a method for determining the characteristics of metallic electrodes in ceramic sensors wherein the metallic electrodes are sprayed as layers and are subjected to subsequent annealing. The purpose of the invention, according to the patent description, is to develop a simple nondestructive and efficient method that allows automation of the sensor acceptance inspection process.
In accordance with the proposed testing procedure, the quantity and distribution of the sprayed gold, due to the fact that it is placed in the protective layer, is determined indirectly. This is done by measuring the layer thickness during the process of manufacturing the electrode, and of respective comparison procedures by means of the eddy current measuring process. To this end the electrode is placed in the magnetic circuit of the coil that is fed with the high frequency current, and the resulting impedance of the coil is measured using the LCR of the measuring circuit. It is noted that the coil can be switched into the resonant circuit by means of a capacitor.
A number of patents deal with the use of two eddy current probes usually operated at different frequencies for measuring the properties of lamellar conducting objects while various procedures for correcting the measurement signals are being used.
The eddy current method is also used for inspecting anomalies in conducting wafers. The excitation and the receiving coils placed at different edges of the wafer initially pass the electromagnetic energy in one direction, from the excitation coil to the receiving coil, then the excitation and the receiving coils change their roles and the energy of the magnetic field is passed in the reverse direction. As a result the defects are detected at an approximately equivalent sensitivity irrespective of the depth of their location in the wafer. The probes generate a periodic magnetic field. The excitation coil and the auxiliary unit direct the electromagnetic field into the wafer being inspected. In an alternative design the auxiliary unit is comprised of a coil controlled by a signal that has a phase and amplitude related to the corresponding signal parameters of the excitation coil. Laminated screens are also used for focusing the electromagnetic energy.
An eddy current measuring transducer containing measuring and compensating probes manufactured in the form of cylindrical induction coils, as well as a measuring circuit has been described. This system is intended for determining the material properties of the object being tested and its geometrical parameters. The measuring transducer operation method includes placing the object at a specified distance from the measuring and the compensating probes, measuring the impedance of the measuring probe at the first and second specified frequencies, determining the material properties, as well as the geometric parameters of the object, on the basis of the impedance measurements, while compensating the temperature influence on the measuring probe by means of the signal being formed by the compensating probe. The compensating probe is spatially smaller of the measuring probe and is located inside the latter. The turns of both probes are co-axial and their geometrical shape is identical. The compensating probe is placed so that the influence thereon of the object being tested is minimal.
The temperature compensation is comprised of subtracting the integrated impedance of the compensating probe from the integrated impedance of the measuring probe. For determining the material properties, as well as the geometric parameters of the object, the latter is initially placed at a distance from the measuring probe that exceeds the probe diameter while the impedance of the measuring probe is being determined. Then the object is brought closer to the measuring probe and its impedance is measured again. The obtained values form a basis for determining the added impedance of the probe which is used for determining the electrical conductance and the geometric dimensions of the object taking into account the correction being used.
The analysis of the patent information and technical literature show that the eddy current method has been widely used for non-contact measurement of the electromagnetic properties and the thickness of layers in lamellar structures and detection of faults in the layers.
However, no patents have been found that deal with the non-contact measuring of the contact resistance between the metallic foil and the thin low-conducting composite coating, the electrical conductance of which can change depending on density, a change in the concentration of its components, granulometric composition (granularity), and homogeneity of the composite mass after mixing of the components.